Elsa Lanchester plays both Mary Shelley and the Bride.
In addition to her role as Minnie, Una O'Connor also appeared in the prologue, as Shelley's maid who is holding the leash as the dogs go off screen.
The Character Died with Him: This is the last Universal Monsters film a living Henry Frankenstein appears in, due to actor Colin Clive's untimely death from alcoholism two years after it was released; the later Frankenstein films are set after the character's in-universe death. However, Frankenstein's ghost appears in the fourth Universal Frankenstein film, The Ghost of Frankenstein, played by a different actor.
This included a re-shot ending where Dr. Frankenstein survives (he was originally meant to die in the laboratory explosion) and some bits that were excised at the behest of the censors, like a scene where Dwight Frye's character murders his uncle and blames it on the monster, or some shots of Elsa Lanchester that were deemed too revealing. Don't expect to ever see these scenes.
Barrier-Busting Blow: An entire courtroom scene was excised in which the Burgomeister called forth anybody who had actually witnessed the murders, but only turned up people who discovered bodies after the fact. After rather scoffingly dismissing the rabble, the Burgomeister himself is attacked by the creature through a window.
Dwight Frye was originally cast in a dual role as both Pretorius's henchman ("Fritz") and the village idiot ("Karl"). Karl's entire subplot was removed from the story, although there's a scene or two where we see Dwight Frye playing Karl (most of the time we see him playing Fritz). So Frye's dual roles become conflated, although the only thing left of the Karl character was the name.
There were numerous other cuts made, including an announcement of Baron Frankenstein's death and Fritz/Karl's discovery of Pretorius's homonculi. There was even some dialogue cut from the Mary Shelley prologue.
Executive Meddling: Due to the Hays Code now being fully in effect, they were now more limited on the violence and number of deaths they could show.
Not Named in Opening Credits: The opening credits bill "The Monster's Mate" with a question mark, and only gives Elsa Lanchester credit for playing Mary Shelley in the prologue. This also acts as a sort of Call-Back to how Karloff was credited in the first movie.
The Other Darrin: The actors for Elizabeth, the Burgomaster, and Hans are replaced.note Elizabeth was played by Mae Clarke in the original Frankenstein and by Valerie Hobson in Bride of Frankenstein due in part to Clarke recovering from an automobile accident. This is a bit jarring since Bride picks up right where the original leaves off and Elizabeth's hair changes from blonde to brunette and the new actress is also considerably younger than Clarke.
Science Marches On: "The human heart is more complex than any other part of the body". Apparently growing brains in a vat is easier than we think.
Troubled Production: The response to Frankenstein during its original preview screenings had been so favorable that Universal shot a new ending in which the monster lived. Director James Whale followed it up with The Invisible Man, which convinced Frankenstein producer Carl Daemmle that only Whale could direct a sequel. But Whale didn't want to, feeling that the original had exhausted the story's potential. Eventually, after Universal let him direct One More River, he gave in.
However, he decided that since the sequel couldn't just be a retread of the first film, a Tone Shift was necessary. The sequel, he declared, would have to be "a hoot". He went through three different story ideas, and more sets of writers, before eventually settling on a story built around a scene in the novel where the monster demands Frankenstein create a mate for him.
The sequel would have the monster actually talk. Although his vocabulary would be limited to 43 words, Boris Karloff thought this was a stupid decision that robbed the monster of his charm. He and Whale were clashing over this as filming began. Colin Clive, who returned as Frankenstein, was for his part plagued by his alcoholism having become worse in the intervening four years. Whale declined to recast the part as he felt that it gave Clive's performance the right over-the-top quality.
Principal photography ran into problems. On the first day, the rubber suit Karloff was wearing beneath his costume filled up with air as he waded into the castle moat. Later that day, he broke his hip, requiring that a stunt double be hired for the rest of the shoot. Clive also broke his leg. The dress that Elsa Lanchester wore to play Mary Shelley in the prologue reportedly took a dozen seamstresses over four months to complete.
Whale shut down production for ten days to wait for the actor he wanted as the Hermit to be available, putting the film behind schedule by that amount of time. It also went $100,000 over budget, a not inconsiderable amount for the time. He finished the final cut only days before the premiere, and had to reshoot the ending. Fortunately for everyone involved, the film made money and is remembered as as much of a classic as the original, if not more.
Original plans for a sequel to Frankenstein were very different. Two other possible plots were conceived, one being the Monster continuing Dr. Frankenstein's research and the other one being Dr. Frankenstein inventing a death ray in the eve of World War I.
The role of Dr. Pretorius was offered to Claude Rains, but he was attached to another project at the time and couldn't participate. They also offered it to Bela Lugosi, but he turned it down.
In the film's original ending, Elizabeth was supposed to be killed and have her heart placed in the Bride (hence why the Bride seems so drawn to Henry during the scene) and Henry was supposed to die in the explosion as well (he can still be seen pinned against the wall in the final cut).
As revealed on Svengoolie, the late midget actor Billy Barty played a baby in Pretorius' collection of little people. His role was cut from the final film, but he can be seen from behind in a wide shot of the collection of little people in jars.